1. Field of the Invention
This invention relates to two-wire telephone systems in general and, in particular, to the transmission of direct current supervisory signals, and audio and modulated carrier signals, between terminals interconnected by a two-wire and/or non-equalized, non-loaded cable.
2. Description of the Prior Art
The increase in cost of labor, material and equipment has substantially increased the desirability of utilizing finer gauge wire for telephone circuits between central offices, and between a central office and subscribers' telephone sets connected thereto. However, the use of finer gauge wire also increases the reactance and resistance of the transmission lines. To control the series impedance of transmission lines, some prior art systems employed inductive and/or a capacitor loading circuit to tailor the impedance and frequency response of individual lines to provide a predetermined characteristic. The use of the loading circuits is expensive, particularly in the labor installation costs. Even after installation, records must be maintained on each of the load coils to provide an indication of the status of the line so that when parties are added or removed from the line the impedance of the line can be properly adjusted to provide the proper characteristics. Hence, the removal of the load coils, as well as the addition of the coils, is an expensive procedure.
Some other prior art systems employ negative impedance repeaters, either in the central office, or in the cable. In such cases, the impedances in the telephone circuit must be matched to provide proper operation. In addition, a filter arrangement is employed to get a flat response in the voice band. However, the negative impedance repeaters often lack sufficient gain, and increasing the gain of these units beyond a preset level may lead to instability. The negative impedance repeaters for the line circuits must be adjusted to the individual line characteristics of each circuit of the cable. The use of negative impedance repeaters requires a constant knowledge of up-to-date line circuit makeup and often time consuming readjustments.
Other prior art telephone circuits have embodied either line equalizing means, or four-wire telephone circuits utilizing hybrid repeaters, or subscriber carrier circuitry.
An object of this invention is to provide a new and improved circuit means for transmitting supervisory and voice, and/or data signals over extended lengths of two-wire transmission lines without requiring the use of equalizers, negative impedance converters, or the like.
Another object of this invention is to provide a new and improved two-wire telephone communication circuit comprising an extended non-loaded, two-wire cable interconnecting telephone terminals which minimizes the deficiencies and high costs of the prior art two-wire and four-wire circuits.
Another object of this invention is to provide a new and improved two-wire telephone circuit comprising an extended two-wire non-loaded cable interconnecting terminals for transmitting DC supervisory, control and ringing signals and voice, and/or data signals in different frequency ranges.
A further object of this invention is to provide a new and improved telephone circuit comprising an extended two-wire non-loaded cable interconnecting any combination of two-wire configured terminals, four-wire configured terminals and a two-wire configured terminal with a four-wire configured terminal for transmitting DC supervisory, control and ringing signals and voice, and/or data signals in different frequency ranges.